iPhone 4: people can't hear me but I can hear them and I can't call out easily. Help??

When I ring people or people ring me, half of the time the person on the other end can't hear me even though I can hear then fine. And it can't be a microPhone issue because the other half of the time it works fine. Also when I try to call out it takes me several attempts as it says "calling" but just remains silent ie no palling tone or anything, and then just cuts off or says call back failed and it is not a signal issue because it happens everywhere and most of the time I have full signal too. What is wrong with it?